1. Field of the Invention
The present invention relates generally to a fluid-filled cylindrical elastic mount, such as a cylindrical engine mount for a front-engine front-drive motor vehicle, and more particularly to improvements in the durability of such a fluid-filled cylindrical elastic mount.
2. Discussion of the Prior Art
There is known a fluid-filled cylindrical elastic mount for effectively damping radially applied vibrations in a certain frequency range, based on resonance of a mass of a non-compressible fluid existing in a restricted passage which communicates with a plurality of fluid chambers defined between an inner and an outer sleeve which are elastically connected to each other by an elastic body interposed therebetween. The restricted passage is tuned to the desired frequency range of the vibrations to be damped. For instance, such a fluid-filled cylindrical elastic mount is used for flexibly mounting a power unit (including an engine and a transmission) on a front-engine front-drive vehicle, so that the body of the vehicle is isolated from vibrations of the power unit.
In such a known fluid-filled cylindrical elastic mount, the inner and outer sleeves are elastically connected to each other by a generally cylindrical elastic body which has a plurality of fluid chambers formed therein in fluid communication with each other through a restricted passage. In this arrangement, the elastic body is inevitably subjected to a tensile force after the elastic mount is installed in its operating position, such that the weight of the power unit or other member to be supported by the relevant elastic mount acts on the elastic body. Therefore, the elastic body of the known elastic mount tends to suffer from relatively low durability, resulting in accordingly short life expectancy of the elastic mount. This means that the known elastic mount discussed above is not suitable as a mounting device which is permanently subjected to the weight of a certain load such as the vehicle power unit indicated above.
There has recently been proposed a fluid-filled cylindrical elastic mount for elastically or flexibly mounting a load member on a support structure, including (a) an inner sleeve attached to one of the load member and the support structure, (b) an outer sleeve attached to the other of the load member and the support structure, and disposed radially outwardly of the inner sleeve with a certain radial spacing therebetween, (c) an elastic body interposed between the inner and outer sleeves for elastically connecting these sleeves, and having at least one pressure-receiving chamber formed therein, (d) means for defining a void formed between the inner and outer sleeves so as to extend in the axial direction of the sleeves, through a portion of the elastic mount not occupied by the elastic body, (e) a closure member having a flexible portion which cooperates with the outer sleeve to define a variable-volume equilibrium chamber within the void, (f) a non-compressible fluid which fills the equilibrium chamber and the pressure-receiving chamber, and (g) means for defining a restricted passage which communicates with each pressure-receiving chamber and the equilibrium chamber.
In the fluid-filled cylindrical elastic mount constructed as described above, the equilibrium chamber communicating with the pressure-receiving chamber through the restricted passage is defined between the closure member and the outer sleeve. This means that the elastic body need not be a cylindrical body, and can therefore be protected from an excessive tensile force due to the weight of the load member, even where the elastic mount is used such that the weight of the load member permanently acts on the elastic body. In other words, the durability of the elastic body and the life expectancy of the elastic mount will not be seriously deteriorated even where the elastic mount is used in such condition.
In assembling the fluid-filled cylindrical elastic mount of the type indicated above, the outer sleeve is usually fitted on the sub-assembly of the elastic body and the closure member, within a mass of the selected non-compressible fluid, so that the pressure-receiving and equilibrium chambers are filled with the non-compressible fluid in the assembling process. The thus assembled elastic mount is subjected, at its outer sleeve, to a drawing operation to give the elastic mount a suitable amount of radially inward pre-compression, and to establish fluid tightness of the pressure-receiving and equilibrium chambers. This drawing of the outer sleeve inevitably causes a relatively high fluid pressure in the equilibrium chamber, which leads to reduced durability of the flexible portion of the closure member.
Described in more detail, the relatively high pressure in the equilibrium chamber in the elastic mount as produced causes a considerably large tensile force permanently acting on the closure member, due to an increase in the fluid pressure in the equilibrium chamber when the elastic mount is installed with the weight of the load member acting on the elastic mount. The pressure in the equilibrium chamber is further increased when an excessively high vibrational load is applied to the elastic mount. The durability of the flexible portion of the closure member defining the equilibrium chamber is necessarily reduced with an increase in the tensile force acting on the closure member.